PHYLUM CHORDATA 



ass 



utriculus (Fig. 350, u) which gives rise to three semicircular 

 canals (ca, ce, cp), and (2) a ventral sacculus (s) bearing an out- 

 growth called the cochlea (I). The bases of the semicircular 

 canals are enlarged into ampulla (aa, ae, ap) containing cells 

 with long sense hairs which record change of position in any 

 direction and are therefore organs of 

 equilibrium. The cochlea of the* sac- 

 culus in higher vertebrates is well 

 developed, contains the auditory sense- 

 cells, and is the true organ of hearing. 



Sound waves are brought to the 

 cochlea in the ears of higher vertebrates 

 by means of the middle ear. This con- 

 sists .of a vibrating membrane, the 

 tympanum, which transmits vibrations 

 to the inner ear with the aid of a chain 

 of three bones. 



In many vertebrates a funnel-shaped 

 fold of skin, which is supported by 

 cartilage, and called the pinna or ex- 

 ternal ear, aids in catching sound waves. 

 In aquatic animals this collecting ap- 

 paratus is not necessary, since the 

 water carries the sound waves to the 

 tissues which transmit them directly 

 to the inner ear. 



The eyes are the most complex of 

 the sense-organs of vertebrates. They 

 arise in part from the sides of the fore- 

 brain and in part from the skin and connective tissue. The 

 principal elements of structure and the method of action may be 

 pointed out by means of a diagram of the human eye (Fig. 351). 

 The eye is nearly spherical. It consists of three concentric coats 

 enclosing transparent substances. The outer or sclerotic coat (Scl) 

 is the white of the eye. It is composed of connective tissue and 



FIG. 350. Semidiagram- 

 matic figure of the left 

 membranous labyrinth of a 

 vertebrate, aa, ae, ap, am- 

 pullae of semicircular canals; 

 ass, apex of sinus utriculi 

 superior; ca, ce, cp, anterior, 

 external, and posterior semi- 

 circular canals; cus, utriculo- 

 saccular canal; de, se, ductus 

 and saccus endolymphaticus; 

 /, recessus sacculi; rec, re- 

 cessus utriculi; s, sacculus; 

 sp, sinus utriculi posterior; 

 ss, sinus utriculi superior ; 

 it, utriculus. (From Wieders- 

 heim.) 



